60 A. E. Yerrill — Study of the family Pectinidce. 



described S. Adamsi, from the West Indies, in deep watei*. JI. jy^isio 

 is from the European coasts. Other species occur in the East Indies. 

 The type is an extinct tertiary species. 



Neithea Drouet, 1824. Type, P. cequicostatus Lam. 



This group agrees in form and sculpture with typical Pecten. It 

 differs chiefly in having a series of transverse denticles and pits 

 along the dorsal border of the hinge-plate. These seem to be 

 homologous with the much finer transverse incisions and denticles 

 found in many living Pectinidae, but are more highly developed. It 

 seems to be scarcely more than a subgenus or section of Pecten. 



All the species are mesozoic fossils. 



Hemipecten Adams aad Reeve, 1849. Type, H. Forhesianus Ad. and R. 



This group includes species with thin, irregular shells attached 

 by the right valve, like Hinnites, but the attachment is effected 

 mainly by a permanent modified byssus. The posterior auricles are 

 nearly obsolete. The byssal notch becomes irregular and nearly en- 

 closed, as in Anomia. I do not know any American species. 



Aviculopecten McCoy, 1855. Type, A. concavus McCoy. 



Shell broad, roundish, more or less inequilateral and oblique, with 

 regular radial sculpture. Auricles unequal, the anterior smaller with 

 a byssal notch. Hinge-plate without a central resilium, but with a 

 ligamental groove along its entire length. The absence of a central 

 resilial pit renders it doubtful whether this genus should be placed 

 in this family. It may belong rather to AvicuUdce. 



The genus is confined to the Palaeozoic rocks. 



Pseudamusium K. and A. Adams. Type, P. exoticum (Chem , Lam.) 



Pseudamussium {pars) H. and A. Adams, 1858 (after Klein); Chenu, 18(32 ; Stoliczka, 

 1871; Ziltel, 1881; Fischer, 1887; T)2i\\(pars) 1886. 



Plate XVII. tigs. 8, 8a. 



The typical species of this group have nearly smooth, round, sym- 

 metrical, closed shells with well-defined, small, straight, obtuse- 

 angled auricles. The valves are nearly equal, and have nearly sim- 

 ple, even margins. The external sculpture consists of small radial 

 striae or riblets, without strong angular ribs and grooves, and it may 

 differ on the two valves ; the margin is not scalloped, or but faintly 

 so, and there are no definite internal ribs. The hinge-plate usually 

 has but one longitudinal fold on each end ; this is feeble and nearly 



